This invention relates generally to methods and apparatus for computed tomographic (CT) imaging, and more particularly to methods and apparatus for conversion of helical projection data into axial projection data for the reconstruction of sectional images.
Cardiac and many other new applications of computed tomographic (CT) imaging demand that multislice CT imaging systems produce high quality images with thinner slice profiles and faster coverage speeds than have been available in the past. To reduce the x-ray dose received by patients, it is desirable to increase the table speed of the CT imaging system. The higher table speed results in a higher helical pitch that often makes it impossible to use full scan algorithms because the data acquired spans a region less than 2xcfx80. One solution is to utilize helical linear interpolation algorithms followed by application of Parker weights to the scan data for image reconstruction.
For example, an axial half-scan algorithm has been used for modes in which the helical pitch is p greater than n, where n is the number of image slices. The performance of such weighting functions (in terms of image quality) is satisfactory when helical pitch is low. However, artifacts and noise increase as the pitch increases.
For example, where a helical view-weight is followed by an axial half-scan weight, the axial half-scan weighting function has been used merely as a passive measure to eliminate data redundancy. Furthermore, a significant portion of data required for image reconstruction has to be obtained by extrapolation. Extrapolation results in increased artifacts and noise.
There is therefore provided, in one aspect, a method for imaging an object utilizing a computed tomographic (CT) imaging system having a rotating gantry, a multislice detector array on the rotating gantry and utilizing at least n greater than 1 rows of detector channels, and a radiation source on the rotating gantry configured to project a beam of radiation towards the multi slice detector array through an object to be imaged. The method includes helically scanning the object with the CT imaging system at a pitch p greater than n to acquire projection data from the n rows of detector channels; applying a combined helical weight and conjugate weight to at least a portion of the acquired projection data to produce virtual projection data compensating for incomplete helical row data of the acquired projection data; and reconstructing an image of the object utilizing the acquired projection data and the virtual projection data.
In another aspect, there is provided a computed tomographic (CT) imaging system having a rotating gantry, a multislice detector array on the rotating gantry and configurable to utilize at least n greater than 1 rows of detector channels, and a radiation source on the rotating gantry configured to project a beam of radiation towards the multislice detector array through an object to be imaged. The imaging system is configured to helically scan the object at a pitch p greater than n to acquire projection data from the n rows of detector channels, apply a combined helical weight and conjugate weight to at least a portion of the acquired projection data to produce virtual projection data compensating for incomplete helical row data of the acquired projection data, and reconstruct an image of the object utilizing the acquired projection data and the virtual projection data.
In yet another aspect, there is provided a computer configured to read projection data obtained by helically scanning an object at a pitch p greater than n utilizing a computed tomographic imaging system using n rows of detector channels, apply a combined helical weight and conjugate weight to at least a portion of the acquired projection data to produce virtual projection data compensating for incomplete helical row data of the acquired projection data, and reconstruct an image of the object utilizing the acquired projection data and the virtual projection data.
In still another aspect, there is provided a computer readable medium having recorded thereon instructions configured to instruct a computer to read projection data acquired by helically scanning an object at a pitch p greater than n utilizing a computed tomographic imaging system using n rows of detector channels, apply a combined helical weight and conjugate weight to at least a portion of the acquired projection data to produce virtual projection data compensating for incomplete helical row data of the acquired projection data, and reconstruct an image of the object utilizing the acquired projection data and the virtual projection data.